Power semiconductor products are often fabricated using extended-drain N or P channel transistors, such as lateral double-diffused metal-oxide-semiconductor (LDMOS) devices for high power switching applications. LDMOS devices advantageously combine short-channel operation with high current handling capabilities and are able to withstand large blocking voltages without suffering voltage breakdown failure. Accordingly, these devices are ideally suited for power switching applications, particularly where inductive loads are to be driven. N channel LDMOS transistors are asymmetrical devices in which a p-type channel region is typically formed in a p-well between an n-type source and an extended n-type drain. Low n-type doping on the drain side provides a large depletion layer with high blocking voltage. LDMOS devices often short the p-well to the source to prevent the p-well from floating, thereby stabilizing the device threshold voltage (Vt).
Solenoid driver integrated circuits and other output drivers often include one or more such LDMOS power transistor devices along with logic and other lower power analog circuitry, wherein the LDMOS transistors are used to provide control outputs to solenoids in automotive or other applications. LDMOS devices have certain performance advantages in such applications, such as relatively low Rdson and high blocking voltage capabilities. Thus, LDMOS devices have been widely used for integrated circuit output drivers requiring blocking voltages in the range of 20-60 volts, and current carrying capability in the range of about 1-3 amps or higher. In addition, LDMOS device fabrication is relatively easy to integrate into CMOS process flows. This allows easy integration in devices where logic, low power analog, or other circuitry is also to be fabricated in a single IC.
For LDMOS transistors designed for higher power applications, a particular design is often a tradeoff between breakdown voltage and on-state resistance (Rdson). Breakdown voltage is often measured as drain-to-source breakdown voltage with the gate and source shorted together (BVdss). Where high breakdown voltage is needed, LDMOS or other drain-extended MOS transistors are often employed, in which the drain region is spaced from the gate to provide a drift region or drain extension in the semiconductor material therebetween. The spacing of the drain and the gate spreads out the electric fields thereby increasing the breakdown voltage rating of the device. However, the drain extension increases the resistance of the drain-to-source current path. In conventional drain-extended LDMOS devices, the Rdson and breakdown voltage are thus generally inversely related, wherein the drain extension causes an increase in Rdson, thus limiting the drive current rating of the device.
Another ongoing trend involves scaling or reducing the size of semiconductor devices. For LDMOS transistors, the tradeoffs between Rdson and BVdss present challenges to scaling efforts, particularly where the design specifications for breakdown voltage and current carrying capability remain the same or increase. Thus, there remains a need for improved MOS transistor devices and manufacturing techniques for scaling LDMOS and other extended-drain transistor devices and for reducing on-state resistance Rdson without significantly impacting breakdown voltage.